Some programming languages (e.g., C) have named functions only, whereas others (e.g., Lisp, Java, and Perl) have both named and unnamed functions. A lambda is an unnamed function, with Lisp as the language that popularized the term. Lambdas have various uses, but they are particularly well-suited for data-rich applications. Consider this depiction of a data pipeline, with two processing stages shown:

Lambdas and higher-order functions

The filter and transform stages can be implemented as higher-order functions—that is, functions that can take a function as an argument. Suppose that the depicted pipeline is part of an accounts-receivable application. The filter stage could consist of a function named filter_data, whose single argument is another function—for example, a high_buyers function that filters out amounts that fall below a threshold. The transform stage might convert amounts in U.S. dollars to equivalent amounts in euros or some other currency, depending on the function plugged in as the argument to the higher-order transform_data function. Changing the filter or the transform behavior requires only plugging in a different function argument to the higher order filter_data or transform_data functions.

Lambdas serve nicely as arguments to higher-order functions for two reasons. First, lambdas can be crafted on the fly, and even written in place as arguments. Second, lambdas encourage the coding of pure functions, which are functions whose behavior depends solely on the argument(s) passed in; such functions have no side effects and thereby promote safe concurrent programs.

Perl has a straightforward syntax and semantics for lambdas and higher-order functions, as shown in the following example:

A first look at lambdas in Perl

#!/usr/bin/perl

use strict;
use warnings;

## References to lambdas that increment, decrement, and do nothing.
## $_[0] is the argument passed to each lambda.
my $inc = sub { $_[0] + 1 };  ## could use 'return $_[0] + 1' for clarity
my $dec = sub { $_[0] - 1 };  ## ditto
my $nop = sub { $_[0] };      ## ditto

sub trace {
    my ($val, $func, @rest) = @_;
    print $val, " ", $func, " ", @rest, "\nHit RETURN to continue...\n";
    <STDIN>;
}

## Apply an operation to a value. The base case occurs when there are
## no further operations in the list named @rest.
sub apply {
    my ($val, $first, @rest) = @_;
    trace($val, $first, @rest) if 1;  ## 0 to stop tracing

    return ($val, apply($first->($val), @rest)) if @rest; ## recursive case
    return ($val, $first->($val));                        ## base case
}

my $init_val = 0;
my @ops = (                        ## list of lambda references
    $inc, $dec, $dec, $inc,
    $inc, $inc, $inc, $dec,
    $nop, $dec, $dec, $nop,
    $nop, $inc, $inc, $nop
    );

## Execute.
print join(' ', apply($init_val, @ops)), "\n";
## Final line of output: 0 1 0 -1 0 1 2 3 2 2 1 0 0 0 1 2 2

The lispy program shown above highlights the basics of Perl lambdas and higher-order functions. Named functions in Perl start with the keyword sub followed by a name:

sub increment { ... }   # named function

An unnamed or anonymous function omits the name:

sub {...}               # lambda, or unnamed function

In the lispy example, there are three lambdas, and each has a reference to it for convenience. Here, for review, is the $inc reference and the lambda referred to:

my $inc = sub { $_[0] + 1 };

The lambda itself, the code block to the right of the assignment operator =, increments its argument $_[0] by 1. The lambda』s body is written in Lisp style; that is, without either an explicit return or a semicolon after the incrementing expression. In Perl, as in Lisp, the value of the last expression in a function』s body becomes the returned value if there is no explicit return statement. In this example, each lambda has only one expression in its body—a simplification that befits the spirit of lambda programming.

The trace function in the lispy program helps to clarify how the program works (as I'll illustrate below). The higher-order function apply, a nod to a Lisp function of the same name, takes a numeric value as its first argument and a list of lambda references as its second argument. The apply function is called initially, at the bottom of the program, with zero as the first argument and the list named @ops as the second argument. This list consists of 16 lambda references from among $inc (increment a value), $dec (decrement a value), and $nop (do nothing). The list could contain the lambdas themselves, but the code is easier to write and to understand with the more concise lambda references.

The logic of the higher-order apply function can be clarified as follows:

1. The argument list passed to apply in typical Perl fashion is separated into three pieces:

   my ($val, $first, @rest) = @_; ## break the argument list into three elements

   The first element $val is a numeric value, initially 0. The second element $first is a lambda reference, one of $inc $dec, or $nop. The third element @rest is a list of any remaining lambda references after the first such reference is extracted as $first.

2. If the list @rest is not empty after its first element is removed, then apply is called recursively. The two arguments to the recursively invoked apply are:
   • The value generated by applying lambda operation $first to numeric value $val. For example, if $first is the incrementing lambda to which $inc refers, and $val is 2, then the new first argument to apply would be 3.
   • The list of remaining lambda references. Eventually, this list becomes empty because each call to apply shortens the list by extracting its first element.

Here is some output from a sample run of the lispy program, with % as the command-line prompt:

The first output line can be clarified as follows:

• The 0 is the numeric value passed as an argument in the initial (and thus non-recursive) call to function apply. The argument name is $val in apply.
• The CODE(0x8f6820) is a reference to one of the lambdas, in this case the lambda to which $inc refers. The second argument is thus the address of some lambda code. The argument name is $first in apply
• The third piece, the series of CODE references, is the list of lambda references beyond the first. The argument name is @rest in apply.

The second line of output shown above also deserves a look. The numeric value is now 1, the result of incrementing 0: the initial lambda is $inc and the initial value is 0. The extracted reference CODE(0x8f68c8) is now $first, as this reference is the first element in the @rest list after $inc has been extracted earlier.

Eventually, the @rest list becomes empty, which ends the recursive calls to apply. In this case, the function apply simply returns a list with two elements:

1. The numeric value taken in as an argument (in the sample run, 2).
2. This argument transformed by the lambda (also 2 because the last lambda reference happens to be $nop for do nothing).

The lispy example underscores that Perl supports lambdas without any special fussy syntax: A lambda is just an unnamed code block, perhaps with a reference to it for convenience. Lambdas themselves, or references to them, can be passed straightforwardly as arguments to higher-order functions such as apply in the lispy example. Invoking a lambda through a reference is likewise straightforward. In the apply function, the call is:

$first->($val)    ## $first is a lambda reference, $val a numeric argument passed to the lambda

A richer code example

The next code example puts a lambda and a higher-order function to practical use. The example implements Conway』s Game of Life, a cellular automaton that can be represented as a matrix of cells. Such a matrix goes through various transformations, each yielding a new generation of cells. The Game of Life is fascinating because even relatively simple initial configurations can lead to quite complex behavior. A quick look at the rules governing cell birth, survival, and death is in order.

Consider this 5x5 matrix, with a star representing a live cell and a dash representing a dead one:

 -----              ## initial configuration
 --*--
 --*--
 --*--
 -----

The next generation becomes:

 -----              ## next generation
 -----
 -***-
 ----
 -----

As life continues, the generations oscillate between these two configurations.

Here are the rules determining birth, death, and survival for a cell. A given cell has between three neighbors (a corner cell) and eight neighbors (an interior cell):

• A dead cell with exactly three live neighbors comes to life.
• A live cell with more than three live neighbors dies from over-crowding.
• A live cell with two or three live neighbors survives; hence, a live cell with fewer than two live neighbors dies from loneliness.

In the initial configuration shown above, the top and bottom live cells die because neither has two or three live neighbors. By contrast, the middle live cell in the initial configuration gains two live neighbors, one on either side, in the next generation.

Conway』s Game of Life

#!/usr/bin/perl

## A simple implementation of Conway's game of life.
# Usage: ./gol.pl [input file]  ;; If no file name given, DefaultInfile is used.

use constant Dead  => "-";
use constant Alive => "*";
use constant DefaultInfile => 'conway.in';

use strict;
use warnings;

my $dimension = undef;
my @matrix = ();
my $generation = 1;

sub read_data {
    my $datafile = DefaultInfile;
    $datafile = shift @ARGV if @ARGV;
    die "File $datafile does not exist.\n" if !-f $datafile;
    open(INFILE, "<$datafile");

    ## Check 1st line for dimension;
    $dimension = <INFILE>;
    die "1st line of input file $datafile not an integer.\n" if $dimension !~ /\d+/;

    my $record_count = 0;
    while (<INFILE>) {
        chomp($_);
        last if $record_count++ == $dimension;
        die "$_: bad input record -- incorrect length\n" if length($_) != $dimension;
        my @cells = split(//, $_);
        push @matrix, @cells;
    }
    close(INFILE);
    draw_matrix();
}

sub draw_matrix {
    my $n = $dimension * $dimension;
    print "\n\tGeneration $generation\n";
    for (my $i = 0; $i < $n; $i++) {
        print "\n\t" if ($i % $dimension) == 0;
        print $matrix[$i];
    }
    print "\n\n";
    $generation++;
}

sub has_left_neighbor {
    my ($ind) = @_;
    return ($ind % $dimension) != 0;
}

sub has_right_neighbor {
    my ($ind) = @_;
    return (($ind + 1) % $dimension) != 0;
}

sub has_up_neighbor {
    my ($ind) = @_;
    return (int($ind / $dimension)) != 0;
}

sub has_down_neighbor {
    my ($ind) = @_;
    return (int($ind / $dimension) + 1) != $dimension;
}

sub has_left_up_neighbor {
    my ($ind) = @_;
    return has_left_neighbor($ind) && has_up_neighbor($ind);
}

sub has_right_up_neighbor {
    my ($ind) = @_;
    return has_right_neighbor($ind) && has_up_neighbor($ind);
}

sub has_left_down_neighbor {
    my ($ind) = @_;
    return has_left_neighbor($ind) && has_down_neighbor($ind);
}

sub has_right_down_neighbor {
    my ($ind) = @_;
    return has_right_neighbor($ind) && has_down_neighbor($ind);
}

sub compute_cell {
    my ($ind) = @_;
    my @neighbors;

    # 8 possible neighbors
    push(@neighbors, $ind - 1) if has_left_neighbor($ind);
    push(@neighbors, $ind + 1) if has_right_neighbor($ind);
    push(@neighbors, $ind - $dimension) if has_up_neighbor($ind);
    push(@neighbors, $ind + $dimension) if has_down_neighbor($ind);
    push(@neighbors, $ind - $dimension - 1) if has_left_up_neighbor($ind);
    push(@neighbors, $ind - $dimension + 1) if has_right_up_neighbor($ind);
    push(@neighbors, $ind + $dimension - 1) if has_left_down_neighbor($ind);
    push(@neighbors, $ind + $dimension + 1) if has_right_down_neighbor($ind);

    my $count = 0;
    foreach my $n (@neighbors) {
        $count++ if $matrix[$n] eq Alive;
    }

    return Alive if ($matrix[$ind] eq Alive) && (($count == 2) || ($count == 3)); ## survival
    return Alive if ($matrix[$ind] eq Dead)  && ($count == 3);                    ## birth
    return Dead;                                                                  ## death
}

sub again_or_quit {
    print "RETURN to continue, 'q' to quit.\n";
    my $flag = <STDIN>;
    chomp($flag);
    return ($flag eq 'q') ? 1 : 0;
}

sub animate {
    my @new_matrix;
    my $n = $dimension * $dimension - 1;

    while (1) {                                       ## loop until user signals stop
        @new_matrix = map {compute_cell($_)} (0..$n); ## generate next matrix

        splice @matrix;                               ## empty current matrix
        push @matrix, @new_matrix;                    ## repopulate matrix
        draw_matrix();                                ## display the current matrix

        last if again_or_quit();                      ## continue?
        splice @new_matrix;                           ## empty temp matrix
    }
}

## Execute
read_data();  ## read initial configuration from input file
animate();    ## display and recompute the matrix until user tires

The gol program (see Conway』s Game of Life) has almost 140 lines of code, but most of these involve reading the input file, displaying the matrix, and bookkeeping tasks such as determining the number of live neighbors for a given cell. Input files should be configured as follows:

 5
 -----
 --*--
 --*--
 --*--
 -----

The first record gives the matrix side, in this case 5 for a 5x5 matrix. The remaining rows are the contents, with stars for live cells and spaces for dead ones.

The code of primary interest resides in two functions, animate and compute_cell. The animate function constructs the next generation, and this function needs to call compute_cell on every cell in order to determine the cell』s new status as either alive or dead. How should the animate function be structured?

The animate function has a while loop that iterates until the user decides to terminate the program. Within this while loop the high-level logic is straightforward:

1. Create the next generation by iterating over the matrix cells, calling function compute_cell on each cell to determine its new status. At issue is how best to do the iteration. A loop nested inside the while loop would do, of course, but nested loops can be clunky. Another way is to use a higher-order function, as clarified shortly.
2. Replace the current matrix with the new one.
3. Display the next generation.
4. Check if the user wants to continue: if so, continue; otherwise, terminate.

Here, for review, is the call to Perl』s higher-order map function, with the function』s name again a nod to Lisp. This call occurs as the first statement within the while loop in animate:

while (1) {
    @new_matrix = map {compute_cell($_)} (0..$n); ## generate next matrix

Higher-order functions such as map encourage the code brevity for which Perl is famous. My view is that such functions also make code easier to write and to understand, as they dispense with the required but messy details of loops. In any case, lambdas and higher-order functions make up the Lispy side of Perl.

If you're interested in more detail, I recommend Mark Jason Dominus's book, Higher-Order Perl.